Hypoeutectic aluminum-silicon alloys contain less than about 12% silicon and have a microstructure consisting of aluminum dendrites in a eutectic. Alloys of this type have a relatively high tensile strength and good ductility. However, hypoeutectic aluminum-silicon alloys lack wear resistance, and as a consequence have not been used in applications where wear resistance is a prime necessity.
Hypereutectic aluminum-silicon alloys, those containing in excess of about 12% silicon, contain precipitated primary silicon particles embedded in a eutectic matrix consisting of continuous phases of aluminum and silicon. Because of the presence of the precipitated silicon particles, alloys of this type have good wear resistance, but the tensile strength, as well as the ductility is less than the hypoeutectic aluminum-silicon alloys.
It has been recognized that the lighter weight and heat transfer properties make aluminum alloys the logical choice as a material for internal combustion engines. Because of this and due to their relatively high tensile strength and ductility, hypoeutectic aluminum-silicon alloys have been used in the past to cast engine components, such as engine blocks and cylinder heads. While it would also be desirable to construct connecting rods of the engine of a hypoeutectic aluminum-silicon alloy because of its relatively high tensile strength, it has been found that these alloys exhibit poor high cycle fatigue strength and do not have an endurance limit, meaning that the alloy will ultimately fail at high cycles. The poor high cycle fatigue strength of hypoeutectic aluminum-silicon alloys is believed to be due to the primary aluminum dendrites that are fixed in the microstructure upon solidification of the alloy and do not change upon subsequent heat below the solidus temperature. Therefore, due to its poor high cycle fatigue strength, hypoeutectic aluminum-silicon alloys have not been considered to be a candidate for connecting rods in an internal combustion engine.
Because hypoeutectic aluminum-silicon alloys have not been considered a good material for connecting rods, it has been erroneously concluded in the past that hypereutectic aluminum-silicon alloys would also be a poor choice for connecting rods, because these alloys have a lower ultimate tensile strength and ductility than the hypoeutectic alloys and it was further believed that hypereutectic aluminum-silicon alloys would follow a similar pattern and have poor high cycle fatigue strength.